Stephan Wildermuth

Stator circulating currents as media of fault detection in synchronous motors

Often found in critical, high power applications, synchronous machines require reliable condition monitoring systems. Large synchronous machines are typically designed with parallel connected windings in order to split the currents in parallel paths, delivering the total power at the terminals. Under ideal symmetrical conditions, no current will circulate between parallel branches of the same phase. However, when a motor fault breaks this symmetry, currents circulate between the branches. Thus, due to the fact that they are only non-zero under faulty conditions, circulating currents potentially represent a sensitive indicator of faulty condition. In this paper, the advantages of using the circulating current between parallel branches of the stator of a synchronous motor as an early indicator of motor faults are shown. Analysis is conducted both through simulation, via the use of finite element methods (FEM), and through experimentation using a specially-designed synchronous machine which allows various fault conditions to be investigated. Through comparison between experiment and simulation, the simulation tool is validated. Furthermore, it is shown that the circulating current is better suited for fault detection than either the branch or the stator current. It is concluded that an improved condition monitoring and protection system for a synchronous machine may be achieved if these currents are monitored.

Condition monitoring of electric motors based on magnetometer measurements

Using micro-sensors in industrial applications is of great interest due to their small size, low-cost and little power consumption. However, the harsh environmental conditions encountered in an industrial environment have so far hindered the widespread use of, for example, MEMS-based sensors. Such sensors are particularly suited for mobile condition monitoring of industrial machinery as short time placement and operation of these sensors is typically unproblematic for many monitoring applications. In this paper we use a miniature triaxial geomagnetic sensor for condition monitoring of low voltage motors. The performance of the magnetometer is studied under conditions encountered in industry. Furthermore the magnetometer is used to measure magnetic fields of an electric motor in a healthy state and in case of a broken rotor bar. By frequency analysis of this data it is demonstrated that the magnetometer measurements can be employed to distinguish between these motor conditions.

Signal processing for electro-optic voltage sensor

We present an electro-optic sensor for accurate AC voltage measurement in high voltage substations. This paper focuses on the novel signal processing method developed to retrieve the applied voltage from the sensors periodic optical signals. The sensor head was presented in an earlier publication. The method is capable of extracting the sensor temperature directly from the optical signals and uses this information for temperature compensation. This enables the sensor to operate with an accuracy of ±0.2% between -40 °C and +80 °C. Compared with earlier solutions, robustness has been improved as the method can now determine the intrinsic birefringence of the optical sensing element and thereby remove a fundamental source adding to the measurement error. This allows relaxing the requirements for high quality optics.

Non-invasive, energy-autonomous and wireless temperature sensor for the process industy

Temperature sensors for application in the process industry are typically mounted in a thermowell. These tubular fittings penetrate the surface of e.g. a pipe and allow to insert a temperature sensing element into the process medium. Although this invasive sensing is state-of-the-art, thermowells suffer from significant drawbacks: Any pipe penetration is a potential cause for leakage and the thermowell itself is prone to corrosion, abrasion and bending stress. Avoiding the thermowell by measuring temperature on the surface of the process container was so far not considered a widespread alternative to invasive sensing. The temperature of the surface is generally not considered representative for the temperature of the process medium. A surface sensor is strongly exposed to the ambient condition and, furthermore, the calibration of such a surface sensor is challenging. In this paper we demonstrate how to overcome these limitations by an optimized thermo-mechanical design and enhanced model-based sensing. In combination with energy autonomous operation and wireless communication our novel non-invasive sensor offers a remarkable flexibility enabling scenarios discussed within the context of internet of things (IoT).

Condition monitoring of synchronous motors based on measurement of circulating stator currents

Condition monitoring based on currents circulating between parallel stator windings of a synchronous motor has been demonstrated. Circulating currents arise from any asymmetries in the motor that influence the air-gap magnetic field such as an eccentric rotor or an inter-turn short circuit. A measurement system based on six individual current sensors has been designed to acquire current data on an industrial motor with high fidelity in order to get a better understanding of the sensitivity of the circulating currents to the different motor states. The selection of appropriate sensors as well as their dielectric layout is discussed in detail. The frequency analysis of the acquired current waveforms yields a rich and complex spectrum, proving the very high sensitivity of circulating currents even to small motor asymmetries.

Infrared temperature sensor for industrial application: Package design for reliable operation in a high voltage generator circuit breaker

An IR temperature sensor for industrial applications has been developed with the focus on achieving the required performance while providing an economical solution. The sensor is based on a thermopile detector supplemented by a specifically designed package to maintain the performance even under harsh environmental conditions encountered in a high voltage generator circuit breaker. Simulations of the thermal layout of the sensor have been performed to optimize the package. The performance of the sensor has been validated by experiments: Several sensors were tested for object temperature from room temperature to 120 °C with a linearity error of less than 3 °C. The accuracy was maintained under realistic ambient temperature cycles between -5 °C and 60 °C. Furthermore EMI and vibration tests have been performed.